Abstract

Premature failures & progressive damage from fatigue - of pump risers at a low lift pumping station located on Lake Mead in Southern Nevada were investigated. A typical pump riser was 30 inches in diameter, 100 to 130 feet long, manufactured from 0.375 inch thick steel and installed in a 54 inch diameter well casing. A pump was installed at the bottom of a typical riser connected by a drive shaft that ran the length of the riser to the electric motor mounted on the pumping station floor. Metallurgical analysis revealed that the mode of failure was fatigue. Stress concentration effects of local weld geometry and a taper in the riser structure were investigated. Strain gages, accelerometers, and ultrasonic position sensors were installed on recently repaired pumps, risers, and motor assemblies to verify analytical predictions and conduct modal analysis. A fatigue analysis found that the 100 foot risers accumulated significantly more damage than the 130 foot rises. The risers swayed at their natural frequency as a result of flow induced vibration of the pump suction. The bending stress was limited in the more flexible 130 foot riser resulting in a much longer fatigue life prediction.